1. Field of the Invention
The present invention relates to an information reproducing apparatus for reproducing digital information recorded on an information recording medium and, more particularly, to an information reproducing apparatus for reproducing digital information by a self-clock scheme.
2. Related Background Art
Conventionally, in reproducing digital information from a recording medium, a reproduction clock synchronized with reproduction data is extracted, and the reproduction data is detected by using the obtained reproduction clock. Methods of extracting such a reproduction clock include a method of recording clock information on a recording medium in advance independently of recorded information, and extracting a reproduction clock from the clock information, and a method called a self-clock scheme, in which a reproduction clock is extracted from a reproduction signal reproduced from a recording medium.
FIG. 1 is a block diagram showing an information reproducing apparatus of the self-clock scheme. Referring to FIG. 1, a reproduction signal reproduced from a recording medium (such as an optical disk) is amplified by a preamplifier 101. The amplified signal is waveshaped by an equalizer 102 to be converted into a signal waveform allowing detection of data. The waveform-equalized reproduction signal is binarized by a binarizing circuit 103 according to a predetermined slice level to be converted into a binary digital signal. This digital signal is input to a PLL circuit 104 and a data separator 105. The PLL circuit 104 compares the digital signal with a clock signal output from an oscillator (not shown), and adjusts the frequency of the clock signal in accordance with the phase difference between the two signals, thereby extracting a reproduction clock almost synchronized with the digital signal. The data separator 105 detects the digital signal by using the reproduction clock to generate reproduction data perfectly synchronized with the reproduction clock.
It has recently been required for an information recording/reproducing apparatus to perform high-density recording of information. As a method of meeting this requirement, an equalization detection method called partial response equalization has been proposed. A partial response scheme will be described in detail later. According to this scheme, data detection is performed, with intersymbol interference being added, so as to change the frequency characteristics of an equalizer output to that in a narrow band, thereby allowing high-density recording of information. However, in order to realize such a scheme, data must be detected by multivalue detection.
FIG. 2 is a block diagram showing an information reproducing apparatus of such a partial response scheme. Assume that PR (1, 1) is employed for partial response equalization. Referring to FIG. 2, first of all, a reproduction signal is amplified by a preamplifier 111. The signal is then subjected to Nyquist equalization in an equalizer 112. Nyquist equalization is equalization to a Nyquist waveform. A Nyquist waveform is a waveform whose signal level becomes "0" or "1" at a data distinguishing point. An output signal from the equalizer 112 is binarized by a binarizing circuit 113. A PLL circuit 114 extracts a reproduction clock from the binarized digital signal, as in the case shown in FIG. 1.
The output signal from the equalizer 112 is also sent to the equalizer 115 to be subjected to waveform equalization by a partial response. In this case, as described above, since PR (1, 1) is employed for partial response equalization, an equalizer 115 performs signal processing of adding an input signal and a signal delayed by one clock period. The reproduction signal having undergone partial response equalization in the equalizer 115 is compared with two slice level in a binarizing circuit 116 so as to be converted into a ternary signal. A data separator 117 detects the ternary signal by using a reproduction clock to detect the reproduction data.
In the information reproducing apparatus shown in FIGS. 1 and 2, if high-density recording of information is performed, the error rate cannot be reduced. This problem will be described in detail below. Assume that in the apparatus shown in FIG. 1, the equalizer 102 is adjusted to optimize PLL control. That is, the waveform equalization ratio of the equalizer 102 is adjusted such that points (edges) crossing the slice level of the binarizing circuit 103 are present at intervals of an integer multiple of a clock period, and dispersion at a clock point is minimized. If, however, the equalizer 102 is adjusted in accordance with the PLL characteristics, an output signal from the equalizer 102 decreases in the amplitude of a short-period signal, resulting in a reduction in margin in the direction of amplitude. Therefore, resistance to level variations at a low frequency in data detection is reduced, and the error rate cannot be reduced.
Assume that the equalizer 102 is adjusted to increase the amplitude of a short-period signal so as to facilitate data detection. In this case, since the positions of edges are shifted depending on patterns, dispersion at a clock point increases. As a result, PLL control becomes unstable, and the error rate cannot be reduced. As described above, if the equalizer 102 is adjusted to optimize one of the PLL characteristics and the data detection characteristics, the remaining characteristics deteriorate, and the error rate increases.
Assume that in the apparatus using partial response equalization in FIG. 2, PLL control is performed by using an output signal from the equalizer 112 for performing Nyquist equalization. In this case, an output signal from the equalizer 112 has a signal waveform like the one shown in FIG. 3. Similar to the case shown in FIG. 1, edges are shifted depending on patterns, and at a clock point increases. In addition, since the frequency characteristics of the equalizer 112 for performing Nyquist equalization are of a high-frequency emphasis type, the S/N ratio decreases. For this reason, PLL control also becomes unstable, and the error rate increases.